The present invention refers to a hub-bearing unit for the wheel of a motor vehicle, including a hub with a radial flange and a central tubular portion having an outer cylindrical surface. A hub-bearing unit of the above-mentioned type is known, for example, from U.S. Pat. No. 6,007,253.
For a better understanding of the state of the art and problems inherent thereto, there will be at first described a hub-bearing unit of known design, reference being made to FIGS. 2 and 3 of the accompanying drawings.
With reference to FIG. 2, a flanged hub 11 has a central portion of cylindrical tubular shape 12. Mounted around the tubular portion 12 is a bearing unit with a dual set of balls 16, 17 radially interposed between a stationary outer bearing race 18 that forms the outer raceways, and two radially inner bearing races 19, 20 located side to side and forming the inner raceways. The races 19, 20 are forcedly mounted on the outer cylindrical surface 12a of the tubular portion 12 of the hub. The inner race 19 located at the axially outer side (or outboard side) rests against a radial shoulder surface 30 of the hub 11. A curved radius 31 joins the outer cylindrical surface 12a to the radial surface 30. The axially inner end of the tubular portion 12 of the hub is plastically deformed, usually by orbital rolling so as to form an edge 23 projecting in a radially outward direction that locks and axially preloads the inner races 19, 20 on the hub, with the inner race 19 abutting the shoulder surface 30.
The base of the flange of the hub is one of the zones where a high concentration of stress occurs. Here, strain deriving from a bending stress reaches values requiring the hub to be previously subjected to an induction hardening heat treatment to improve structural strength of the hub.
The heat treatment inevitably deforms the surface of the hub on which the inner bearing race 19 is to be forcedly mounted. It is therefore necessary to carry out a further grinding treatment to eliminate roundness defects of that surface of the hub in order to attain a good forced coupling between the hub and the inner bearing race 19. The heat treatment and the subsequent grinding considerably affect manufacturing costs of the bearing unit.
However, it is necessary to carry out the above-mentioned heat treatment in order to avoid another drawback. As the steel of the hub is more yieldable than the hardened steel which the inner race 19 is made of, and the radial interface surface with the shoulder 30 has a very small area (see FIG. 3), the axial forces generated during rolling of the edge 23 are transferred to the shoulder surface 30 of the hub, and would yield in this surface—if not treated—a plastic deformation or impression that becomes deeper during the life of the bearing. As a matter of fact, in the absence of a heat treatment, when the hub-bearing unit undergoes high axial loads, the axial preload is lost and an axial play appears, causing jerks that are repeated with every turn of the wheel and that tend to form cracks in the hub.